1. Field of the Invention
This invention relates generally to the field of current share busses designed to enable a load current to be shared equally between multiple power modules, and more particularly to a digital implementation of an interface for such a bus.
2. Description of the Related Art
It is often advantageous to implement a power system using a plurality of individual DC-to-DC or AC-to-DC power modules or supplies connected in parallel. (“Power module” in this context refers to a voltage/current converter, not to the ultimate source of electric current such as a battery or generator). Unlike a single module power source, a multi-module power system can provide for failure recovery if one module ceases to operate. Furthermore, simply supplementing the design with additional power modules may increase the total current capacity of a multi-module system. Often such power systems are used in telecommunications equipment and other equipment requiring a reliable source of power, e.g., matrix switches and industrial controllers.
The total current delivered to a load from a power system having multiple power modules configured in parallel equals the sum of the currents delivered by each individual module. If one module delivers a greater amount of current, that module will dissipate more power and therefore become hotter than the other modules. Higher operating temperature normally yields reduced reliability of the overall power system. Therefore, it is preferable that the total load current be evenly distributed among the parallel-connected power modules.
FIG. 1a illustrates a power system 10 using multiple power modules 12, 14, 16, 18 configured in parallel supplying power to a load 19 connected to an output node 20. Each module accepts an input voltage VDD and provides an output current I0, I1, I2, I3 to output node 20. If the current supplied by the power system is evenly divided among the power modules, each module will deliver an equal amount of power, and no one module will be driven to an extreme that may cause power conversion inefficiencies, power module degradation or premature power module failure.
To evenly distribute the power load, an external controller may be used to sense and adjust each module's current output. Alternatively, the power modules may be designed to communicate among each other and self-regulate their output power. For example, a power system may be designed such that each module communicates its output current to other power modules and each module adjusts its output based on the received signal. One way in which this communication may be effected is with the use of a “current share bus” 22. Here, each power module is connected to the share bus which may be, for example, a single wire providing an analog voltage signal relative to a common ground of power system 10. In operation, each power module attempts to raise the voltage on bus 22 to a value indicative of the current supplied by that power module. The power module providing the greatest current to the load overrides the voltage provided by the other power modules. The voltage level on the shared bus therefore corresponds to a level indicating the current supplied by the power module providing the most current.
As well as providing a voltage indicative of a power module's output current, each power module also monitors current share bus 22 to determine the maximum current supplied by any one of the other power modules. If each power module is providing the same amount of current to the load, the voltage applied to the bus by each module is equal to the voltage monitored by each module from the bus. Any power module providing a level of current below that which is indicated on the bus will detect that at least one module is providing more current, and is arranged to incrementally increase its output voltage until its output current equals the current indicated on the bus. In this way, each of the modules will increase its output current in an attempt to track the output current supplied by the module providing the most current.
FIG. 1b is a simplified schematic showing one means by which a power module (12) could be interfaced to a single-wire current share bus (22) that carries a shared analog signal representing an averaged signal; an example of this approach is found in the ADM1041 Secondary-Side Controller with Current Share and Housekeeping IC from Analog Devices, Inc. An analog current sense signal 26 received from module 12 represents the sensed current. Signal 26 is coupled to an error amplifier 30, the output of which (31) drives the share bus; a diode 32 connected in series with output 31 ensures that the error amplifier can only increase the voltage on the bus.
The voltage on bus 22 is coupled to the bottom side of an offset voltage 36, the top side of which is connected to amplifier 30. Offset voltage 36 ensures that when the power module is a “slave” device—i.e., a power module which is not outputting the highest output current— its current sense information is always less than that associated with the “master” power module; without such an offset, there may be no clear “master”, and the system may be prone to “hunting” for one. The voltage on bus 22 is coupled to power module 12, which causes its output current to adjust as needed to make it approximately equal to that of the master module.
Another approach is described in U.S. Pat. No. 6,788,036 to Milavec et al. Here, a power module's interface circuitry puts a pulse-width modulated (PWM) signal onto the current share bus, with the pulse width being proportional to the current sense value.